1. Field of the Invention
The present invention concerns the preparation of a material having piezoelectric properties through dipole orientation of a poled dielectric, and in particular the preparation of a piezoelectric polymer, as well as a device for carrying out such process.
2. Description of the Prior Art
Numerous materials of various types present piezoelectric properties or are adaptable to present such effects after being subject to appropriate processing. Polarized ferro-electric ceramics or monocrystalline quartz, for example, may be cited. More recently, piezoelectric polymers have been introduced. These piezoelectric polymers are generally used in the form of planar films metallized on tow faces, the film may be mechanically oriented, and in any event the film is polarized by applying an intense electric field (of about 1 MV/cm) at a temperature equal to or higher than ambient temperature. As a result of this processing, the polymer films acquire pyroelectric and piezoelectric properties allowing them to be used as transducers. These planar films can be used in the following applications: pressure sensors, displacement sensors (microphone, stress gauge, etc...) and temperature sensors (pyroelectric sensor for intrusion detection, for infra-red photography). It has been proposed to use these materials, after heat forming or molding into suitable forms, in order to produce earphone, loudspeaker or microphone membranes. Such transducer devices are described, for example, in U.S. Pat. No. 4,384,394.
Among polymers which may be used for such purposes are by way of example, vinylidene polyfluoride (PFV.sub.2), vinyl polychloride (PVC), vinyl polyfluoride (PVF) and copolymers, such as poly (tetrafluoride/ethylene P:(TrFE-VF.sub.3) vinylidene fluoride: P(TFE-VF.sub.2) and poly (trifluoroethylene-vinylidene fluoride.
The dipole orientation in the materials, induced by use of an electric voltage generator, induces stable piezoelectric and pyroelectric properties at least in the temperature range lower than their Curie temperatures. This orientation requires the use of intense electric fields such as indicated hereinabove, such fields being themselves of the same value as the breakdown field. This means that there is a high probability of breakdown during polarization, thereby leading to short-circuiting of the voltage generator, when said generator is applied between electrodes deposited on the dielectric. In order to overcome this drawback, it has been proposed in the prior art to use gaseous electrodes, such as air ionized by a needle.
If the dielectric breaks down locally, which event is characterized by a conductive path between its two faces, the generator is not short-circuited due to the high impedance of the plasma and thus the areas outside the short-circuited zone remain subjected to the electrid field.
However, this method presents several drawbacks.
The surface voltage obviously cannot exceed the potential of the needle, thereby limiting the dielectric thicknesses to be polarized. It is in fact difficult, due to hydrodynamic instabilities that appear in the plasma, to exceed needle voltages beyond about 20 killovolts. If the polarization field necessary for a given dielectric is for example 1 MV/cm, the maximum thickness that can be polarized is 200 micrometers.
The plasma created by a needle contains a very wide spectrum of charged bodies; electrons, ions, complexes, and neutral nitrogen, which are generally very reactive. The first consequence is that the field in the plasma can confer upon certain of the charged bodies sufficient energy for them to be implanted in the dielectric. This dielectric therefore acquires an excess charge (electret state); said excess charge impairs the dipole orientation since the associated field of charged bodies tends to break the regularity of the polarization field and thus generates a non-uniform remnant polarization in the thickness of the dielectric. The second consequence is that the reactivity of the plasma causes erosion of the surface of the dielectric; oxidation, ruptures of the macromolecular chains with or without cross-linking, and such chemical erosion then can impair the dielectric's stability, especially when it is relatively thin.
The present invention is aimed at overcoming the drawbacks of the processes of the prior art recalled hereinabove.
In the process according to the invention, a dipole polarization is carried out by a charged jet of small liquid drops or droplets. This process, while eliminating the drawbacks set out hereinabove, presents the supplementary advantage of simultaneously performing an electrode deposit on the material. In a supplementary alternative embodiment of the invention, the process allows the continuous polarization of the material which is used in the form of a strip or film.
The object of the invention is thus to provide a process for preparing a material having piezoelectric properties as a result of a dipolar orientation of a poled dielectric material, which material acquires the said piezoelectric properties when subjected to an electric field of determined amplitude. The electric field is created between two superficial zones of an object brought to different electrical potentials. The process comprises the steps of exposing at least one of the said superficial zones to an electrically charged jet of droplets, the exposure time is selected so that the charged droplets are accumulated on each exposed superficial zone thus increasing the potential difference between the superficial zones up to the determined electric field amplitude.
A further object of the invention is to provide a device for carrying out the said process.